Microtunnelling system and apparatus

ABSTRACT

A microtunnelling apparatus and system that includes an external drive system having rotational and linear thrust drive means, a drill head section having drill rotor and drill rod and connecting to intermediate drill rods allowing extension of the boring hole created by the drill head section driven by the drive system. The drill head includes a modular construction having a plurality of circular disc like elements, a bearing module, a steering module, a spacer module, and a mounting module, for axial alignment and abutment and mounting within a cylindrical steering shell. Directional steering of the drill head includes a plurality of substantially radially extending channels in steering module, each with an hydraulically movable protuberance movable by control means to redirect the outer steering casing and thereby redirect the drill head section mounted on the distal end of the drill rods.

FIELD OF THE INVENTION

This invention relates to underground boring and more particularly to animproved microtunnelling system and apparatus.

In this document “microtunnelling” is considered to comprise trenchlesshorizontal boring of a bore of the order of 600 millimetres and less.

BACKGROUND OF THE INVENTION

Modern installation techniques provide for underground installation ofservices required for community infrastructure. Sewage, water,electricity, gas and telecommunication services are increasingly beingplaced underground for improved safety and to create more visuallypleasing surroundings that are not cluttered with open services.

Currently, the most utilised method for underground works is to excavatean open cut trench. This is where a trench is cut from the top surfaceand after insertion of piping or optical cable is then back-filled. Thismethod is reasonably practical in areas of new construction where thelack of buildings, roads and infrastructure does not provide an obstacleto this method. However, in areas supporting existing construction, anopen cut trench provides obvious disadvantages, major disruptions toroadways and high possibility of destruction of existing infrastructure(i.e. previously buried utilities). Also, when an open cut trench iscompleted and backfilled the resultant shift in the ground structurerarely results in a satisfactory end result as the trench site oftensinks. Open trenches are also unsafe to pedestrians and workers.

Another concept employed for underground works is that of boring ahorizontal underground hole. Several methods employ this philosophy asit generally overcomes the issues of disruption to roads andinfrastructure as described for open cut trenches however even thesemethods have their inherent problems.

One method is horizontal directional drilling (HDD). In this method aboring device is situated on the ground surface and drills a hole intothe ground at an oblique angle with respect to the ground surface. Adrilling fluid is typically flowed through the drill string, over theboring tool, and back up the borehole in order to remove cuttings anddirt. After the boring tool reaches a desired depth, the tool is thendirected along a substantially horizontal path to create a horizontalborehole. After the desired length of borehole has been obtained, thetool is then directed upwards to break through to the surface. A reameris then attached to the drill string, which is pulled back through theborehole, thus reaming out the borehole to a larger diameter. It iscommon to attach a utility line or other conduit to the reaming tool sothat it is dragged through the borehole along with the reamer. A majorproblem with this method is that the steering mechanism is extremelyinaccurate and unsuitable for applications on grade. The stop and startaction utilised by the operator results in a bore that is not completelystraight. The operator has no way of knowing exactly where the hole goeswhich can result in damage to existing utilities. This could pose asafety threat particularly if the services in the area are of a volatilenature.

Another method is the pilot displacement method. This method uses adrill string pushed into the ground and rotated by a jacking frame. Atheodolite is focused along the drill string as a point of reference tokeep the line on grade. This system is not accurately steered. The slanton the nose is pointed in the direction of intended steering. Theposition of the head is monitored through a total station with a gradeand line set and measuring this point against a target mounted in thehead of the pilot string. If the ground conditions are homogenous andthe conditions absolutely perfect, it will produce a satisfactory bore.Unfortunately this is rarely the case. Ground conditions are generallyvariable the pilot tube will tend to steer towards whichever groundoffers the least resistance irrespective of the direction in which youare the steering. As the drill strings are generally short, the time todrill is often slow with repeated connections making the processtedious. Once the bore reaches the reception shaft augers are attachedand pulled back along the bore to displace the spoil into the receptionshaft. This then has to be manually removed which is time consuming.

Slurry style microtunnelling utilises slurry reticulation to transportspoil removal throughout the installation process. Two lines are fed viaa starting shaft along the bore. The pipes are jacked via a hydraulicjacking frame into the hole. Water is forced along the feed pipe to thecutting face where the spoil slurry of rock and mud is forced back alongthe return pipe. Whilst enjoying a good degree of accuracy, this systemrequires a structural shaft that needs a massive amount of force to pushthe pipes. This results in a large, expensive jacking shaft pit that istime consuming to build. The sheer weight and size of the componentsmake them slow to connect and cumbersome to use. If the unit becomesdamaged or stuck in the bore, the only method available to retrieve theunit would be to dig down onto the drill head location.

In one form of boring machine shown by US Patent Application No.US2004/0108139 to Davies and corresponding to Australian Patent2003262292 there is disclosed a micro tunnelling machine having atunnelling head with a boring bit which is forced in a horizontaldirection by an hydraulic thruster. The direction of the head is laserguided. The beam strikes a target in the head and a camera relays animage of the target to an operator located at the tunnel entrance. Theoperator adjusts the direction by admitting water and draining waterfrom a pair of rams inside the head, which move the boring bit up anddown or left and right. A semi automatic version is disclosed in which amicroprocessor adjusts the direction until the operator assumes control.In particular the invention is claimed to be a guidance system for theboring head of a micro-tunnelling machine of the type which bores in aselected direction and inclination using laser beam guidance having theendmost part of the drive to the boring bit adjustable in two directionsat 90°, wherein, the endmost part of the drive has a target for thelaser beam, means to convey an image of the target and the laser strikeposition thereon to an operator situated remotely from the boring headand input means for the operator to adjust the direction of the endmostpart of the drive.

The major approach of the directional control of the disclosed apparatusof US Patent Application No. US2004/0108139 to Davies is to have thedrive shaft connected at its end distal to the cutting edge in a mannerthat allows the drive shaft to move as required and to allow the cuttingelement to be redirected to correct position as determined by the lasercontrolled directional system. However this form of apparatus places allthe strain on an elongated movable drive shaft retained by cylinders andtherefore readily increases the risk of breakage. There is clearly aneed to provide an improved system to decrease chance of breakage of thedrill head components.

It can be appreciated that present methods of underground tunnelling arecumbersome, inaccurate; and require repeated halting of boringoperations due to waste removal and heating effects. Moreover, there isan inherent delay resulting from replacement of parts of conventionalboring systems since it usually requires the boring tool to be recoveredfrom the site and returned to the assembly factory. Recovery in itselfcan be cumbersome and expensive particularly if a new vertical accesshole is required to recover the tool. This could damage the road orservices under which the bored tunnel is extending. Further presentsystems are unable to accurately remain on fixed boring direction, whichare often needed when a buried obstruction is detected or changing soilconditions are encountered.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided an apparatus andmethod for underground boring on grade more particularly to an improvedmicrotunnelling system and apparatus.

In this document “microtunnelling” is considered to comprise trenchlesshorizontal boring of a bore of the order of 600 millimetres and less.This is particularly relevant to the insurgence of pipes of the order ofaround 300 millimetres.

The drawbacks of current microtunnelling technology are significant andhave been overcome or are at least ameliorated by the current inventionincluding one or more of the following improvements and otherimprovements as will be understood from the description.

A first fundamental improvement is the use of an external casing withflow channels therein and the drive rod mounted therein and allows forall cabling and hosing to be mounted in an external cavity, whichthereby allows for continuous cabling over a plurality of encasedintermediate drill rods.

A second fundamental improvement is the incorporation of the drivelinewithin the vacuum chamber. Incorporating the rotation within the vacuumachieves multiple goals. Firstly, the vacuum area can be dramaticallyincreased and so maximize the machines ability to remove spoil and insuch increased productivity. Secondly, the rotation component of thedrill rod generates heat. The removal of this heat from the laser areais critical to laser accuracy. By combining the rotation into the vacuumarea, any heat generated is immediately removed and the laser thereforeis unaffected.

A third fundamental improvement is the steering mechanism of the encaseddrill rod using radially protrusions engaging steering shell to directthe drill head and prevent any undue force on the drill head centrallymounted within the casing.

A fourth fundamental improvement is the modular structure of the drillhead by a plurality of disc like modules that can be created by directexternal etching, drilling or casting or the like and be combined incylindrical shells to form a readily assembled drill head.

A fifth fundamental improvement is the modular components of the drivemeans that allows for differing rotational units to be used with athrust unit that provides linear pull as well as push capabilities. Thisallows matching of rotational units to material being bored and size ofpipe being inserted and further allows for reverse reaming to a largerdiameter after initial bore has been accurately drilled.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention is more readily understood an embodimentwill be described by way of illustration only with reference to thedrawings wherein:

FIG. 1 is a perspective view of a drive means of a microtunnellingsystem and apparatus in accordance with the invention including a thrustmodule and rotation module mounted on a rack system and furtherincluding a vacuum for assisting return slurry;

FIG. 2 is a perspective exploded view of a drill head able to be drivenby the drive means of FIG. 1 for use in the microtunnelling system andapparatus in accordance with the invention;

FIG. 3 is a front view of an enclosed drill head with front cuttingmeans able to be driven by the drive means of FIG. 1 for use in themicrotunnelling system and apparatus in accordance with the invention;

FIG. 4 is a cross sectional view of the enclosed drill head with frontcutting means of FIG. 3 through section A-A;

FIG. 5 is a cross sectional view of the enclosed drill head with frontcutting means of FIG. 3 through section B-B;

FIG. 6 is a cross sectional view of the enclosed drill head with frontcutting means of FIG. 3 through section C-C;

FIG. 7 show front and rear perspective views of the steering module ofthe drill head of FIG. 2;

FIG. 8 is a side view of the of the steering module of FIG. 7 and across sectional view through section B-B;

FIG. 9 show front and rear perspective views of the bearing module ofthe drill head of FIG. 2;

FIG. 10 is a side view and a cross sectional view of a drill shaft;

FIG. 11 show front and rear perspective views of the front bearing bushof the drill head of FIG. 2;

FIG. 12 is a side view of the of the front bearing bush of FIG. 11 and across sectional view through section A-A;

FIG. 13 is a cross sectional view of the enclosed drill head showing thepressure fluid path through the modules to the bearing module and thefront bearing bush supporting the front cutting arm;

FIG. 14 is a perspective view of a drive rod for extending between thedrive means of FIG. 1 and the drill head of FIG. 2

FIG. 15 is a perspective reverse view of the drive rod of FIG. 6;

FIG. 16 is are end views of the drive rod of FIGS. 14 and 15 showingmating male and female ends; and

FIG. 17 is a perspective detailed view of the drill rod of FIGS. 14 and15 showing the toggle locking mechanism.

FIG. 18 is a rear perspective view of a vacuum assisted precision reamershowing the connection means to the drill rod and rearward facingcutting face.

FIG. 19 is a front perspective view of a vacuum assisted precisionreamer of FIG. 18 showing the connection means to the product pipe to beinstalled.

FIG. 20 is a rear perspective view of a vacuum assisted precision reamerof FIG. 18.

FIG. 21 is a cross-sectional view through section A-A of FIG. 20 of avacuum assisted precision reamer of FIG. 18 showing the internalpressure fluid passages, vacuum cavity, air channel, input drive shaft,planetary gear set, cutter hub and bearing.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings there is shown a microtunnelling apparatus andsystem that comprises a drive system (11), a drill head section (20) andintermediate drill rods (41) allowing extension of the boring holecreated by the drill head section driven by the drive system.

The drive system (11) as shown in FIG. 1 includes a power source and atrack system for allowing limited linear drive of the power source. Thetrack system includes a rack and pinion gearing system (12) to allowmaintained linear thrust pressure along the length of the track. Thepower source includes a hydraulic thrust module (13), which reciprocatesa rotation module (14) housed in the thrust box in the launch shaft. Theproduct pipe can be either pushed or pulled into place for pipelinecompletion.

To the front of the rotation module (14) is attached encasedintermediate drill rods (41) such as shown in FIGS. 14 and 15.

Attached to the distal end of the last intermediate drill rod (41) isattached a drill head (20) shown in exploded view in FIG. 2 and in crosssectional views in FIGS. 4, 5, and 6. As such a drill rotor assembly(21) connected to the end of the drill shaft or drill rod (22) andconnecting to intermediate drill rods (23) form a continuous drillstring that is driven by the external drive means (11) comprising thehydraulic thrust module (13), reciprocating a rotation module (14) andlinearly movable on the rack and pinion gearing system (12).

The casing (42) of the intermediate drill rods (41) and the casing ofthe drill head (20) formed by the steering shell (M6) and the rear shell(M5) form a continuous covering of the continuous drill string withinternal defined continuous bores or channels. In particular a vacuumchannel (51), as shown particularly in FIG. 16, can be formed by anumber of continuous cavities extending along the length of theintermediate drill rods (41) to the drill head (20). This vacuum channel(51) has vacuum seals at connecting female end (46) to maintain vacuumbetween longitudinally engaged and aligned intermediate drill rods.Within this vacuum channel 51 is located the connecting intermediatedrill rods (41). A separate air channel (52) is formed by a separatenumber of continuous cavities extending along the length of theintermediate drill rods (41) to the drill head (20). This forms a linearchannel within which the controlling laser can penetrate to the drillhead (20). By the separation of the heat generating drill rod (22) tothe linear laser channel and the cooling effect of the return slurryalong the vacuum channel (51) creates a highly effective and accuratesteering mechanism.

The microtunnelling system and apparatus further includes:

-   -   a) drill head with fluid bearing bush and modular construction    -   b) enclosed drill rods with internal cooling system    -   c) pullback extraction reamer    -   d) rack and pinion thrust module with rotation unit    -   e) rod loading system    -   f) microprocessor control system.

In use upon excavation of a launching shaft, the base of the shaft wouldbe prepared for the installation of the drilling machine. The shaftwould typically have a pipe invert start point already marked and a linesurveyed. A laser would be set up in the shaft at the extreme rear online and grade. Thick boards are typically placed along the base of theshaft horizontally on grade. The microtunnelling drive means (11)including thrust module (13) and rotation unit (14) is lowered into theshaft and set up on line and grade.

The drill head (20) is lowered into the shaft and data, hydraulic andpressure fluid lines (44) are attached to the drill head (20). The drillhead size and ground conditions are entered into the control panel whichselects appropriate parameters for drill thrust speed and force, drillrotation speed and torque, vacuum flow and pressure, and pressure fluidflow. The drill head is attached to the vacuum thrust adaptor mounted onthe rotation unit. Once set in launch mode, the vacuum unit is startedand the pressurised drill fluid is actuated to eject at the drill face.The drill head is launched into the earth face.

The hole is cut via a combination of rotating cutting tooling andassisted by ejecting pressurised fluid. This pressurised fluid flow,which also acts as a fluid bearing, is shown in bold in FIG. 13. Whilstdrilling, the drill head (20) is thrust into the ground with theslurry/spoil being vacuumed up back into vacuum pipe (15) into a wastetank for removal. Once the drill head is completely in the ground thethrust, rotation, vacuum and pressure fluid is stopped. The drill headis detached from the vacuum thrust adaptor, and the thrust trolley withrotation unit return to the starting position.

Once in the start position an intermediate drill rod (41) is loadedeither manually with a crane or via the use of the automated rod loader.Once the drill rod is sitting in the bed of the thrust module the thrusttrolley and rotation unit are started at low speed, low thrust and lowtorque respectively to engage the drill rod. The rod engagement isautomatic in that the drill rod has self-aligning pins (48) thataccurately aligns the rod to both the drill head and the drill machine.Upon full alignment and further forward travel, the self-locking toggles(shown in detail in FIG. 17) engage behind the locking pins to affect asolid connection. Control hoses and cables (44) are inserted into theconcave cavity (43) of the outer cover or casing (42) encasing the drillrod (23). Vacuum and pressure fluid resume with the drilling processreverting to preset drilling speed, thrust and torque. This process iscontinued until the final bore end point is reached.

Operation of the microtunnelling machine is performed remotely via acontrol box, which displays all the current pressure and speed settings.The control box is computerised and integrates the control of thesteering, thrust module, rotation unit, vacuum unit and the pressurefluid. The operator can adjust any of the parametric settings toperfectly suit the current ground conditions. Both the drilling processand the steering process can be automated via the use of integratedcomputer software and can also be manually controlled. Throughout thedrilling process the drill position is monitored via the laser hitting atarget positioned in the drill head (20) and viewed through the use ofclosed circuit television (CCTV) so that the operator or softwarepackage constantly steers the drill head to keep the laser in the centreof the target.

Once the bore is complete there are three options; progress the drillrods into the reception shaft whilst inserting jacking pipes, pull backto the launching shaft whilst trailing a pipe directly behind it, orremove the drill rods prior to pipe insertion.

Currently, the microtunnelling industry only allows for forwardexcavation. The current invention is the only system of microtunnellingthat incorporates precision back reaming. As shown in FIGS. 18 to 21there is provision for the drill head (20) to be replaced by a backreamer (60) that is similarly connected to the intermediate drill rod(41) and driven by the drill string and external drive means. Howeverinstead of forward facing drill rotor assembly (21) of similar diameterto the drill head (20), instead there is a rearward facing reamingassembly (61) of larger diameter to the intermediate casing (42). Thepipe can be installed by back reaming and attaching pipe to opencylindrical end housing (65) mounted at the very end of the back reamer(60). Thereby as the back reamer (60) is drawn back by the drive means(11) while undertaking rotational drilling with rearward facing reamingassembly (61) of larger diameter, a pipe of same or smaller diameter isdrawn along and laid in the enlarged bore.

Back reaming allows use of low cost reamers to open the hole fordifferent pipe size installations. Back reaming also utilises one sizedrill head and drill rod for each thrust module which in turn simplifiesthe rod loading process and reduces overall equipment cost.

Looking at the apparatus in further detail the system includes:

-   -   Guidance system with a laser striking a target, which is        monitored to constantly maintain an accurate position.    -   Vacuum: Use of vacuum allows for clean operation, fast        extraction minimising regrind and Vacuum also reduces volume        area occupied by extraction unit    -   Pressure Fluid: Allows for enhanced cutter life whilst creating        greater option via the use of drill fluid when dealing with        different drill conditions.    -   Drill rods: providing the ability to push or pull means that we        can cut in both directions. This allows the machine to        essentially drill a pilot hole accurately on the thrusting        forward of the line and then cut back or open the hole as you        pull back. As the line and grade of the hole is already        determined the tooling required is simplistic and inexpensive        which allows the machine to be more versatile through a large        range of hole sizes at minimal cost. Pulling back in        microtunnelling is unique. By only using one sized drill rod for        each unit the jacking frame can be customised to automate the        loading and unloading of the drill rods. With automated loading        and unloading of drill rods the system reduced the need for man        entry whilst operating. This enhances safety on the worksite.

The thrust module, which is installed in the launching shaft, canprovide 300 kN force for thrust and pullback of 2.5 metre stroke withina longitudinal space of 3.0 metres. The thrust module uses rack andpinion gearing for increased stroke to retracted length ratio. Itprovides a high load capability with positive force. Pressure, force andspeed are fully adjustable for both thrust and pull back and have aprogrammable stroke with adjustable limit stops for the trolleyassembly. Overall the thrust module allows fast drop in boxes for therotation unit.

A variety of rotation modules can be selectively utilised with the onethrust module according to the requirements. Rotation modules ideallycater for one drill diameter, by maximising available hydraulic power,rotating at ideal speeds (rpm) by maintaining optimum cutting facesurface speeds (m/min) to best utilise working range of tungsten andcarbide cutting inserts, and by maintaining the most desirable cutface/vacuum area ratio. Other sizes of rotation modules can also be usedbut with less efficiency.

Each rotation module comprises its own hydraulic motor (low speed/hightorque, high speed/low torque, two-speed automatic selective unit, orother) coupled through a drive train assembly (chain and sprockets,simple gear box, planetary gearbox, or other) to rotate a drive shaftwith a hexagonal end, which is to be coupled to the drill string insidethe drill rods.

Each rotation module also includes a Vacuum thrust adaptor forconnection with drill rods. This vacuum thrust adaptor incorporates thefeatures suited to each drill rod, being vacuum sealing method, drillrod alignment, drill string torque transmission connection, thrust faceand pullback connection. The Vacuum thrust adaptor also houses anyhydraulic clamping and disconnection mechanisms for drill rods.

The microtunnelling machine targets extremely precise small diametertrenchless pipe installations particularly <600 mm and more particularly<300 mm. This is achieved by tracking a laser striking a target in thedrill head, which is monitored via CCTV in the drill head and thensteered accordingly to maintain line and grade. A unique fluid bushassembly transmits water and thrust to the rotating cutting face, wherethe pressure water and subsequent cutting spoil are mixed to a slurryfor removal by vacuum extraction.

The drill head utilises a unique radial steering system capable ofdirectly variable directional changes to continually and precisely cutthe bore hole. The drill head is progressed through the ground byconnecting subsequent drill rods between the drill head and thrustmodule until final bore length is achieved. These drill rods are eitherencased or open and combine rotation shaft/drill string, vacuum, air andcontrol channels providing mechanical and control workings. Hydraulics,water and data is remotely controlled and utilised by the operator atthe remote control panel and conveyed by cables and pressure hoses.

The front cutting rotor assembly consists of tungsten, carbide or othersintered hard metal inserts housed both axially and radially on avariety of face styles. The shape of the front cutting face variesremarkably with ground conditions, and can be flat, piloted or conicalin shape and is built to suit.

All front cutting rotors are designed so that cuttings large enough topotentially block drill head vacuum cavity are kept ahead of cutters forfurther processing (mixing, cutting, grinding or shattering). Oncecuttings are small enough, they are permitted past the cutter face forvacuum extraction.

A clay cutting face will have a multitude of spokes (range from 3 to 6)possibly connected together again to an outer rim. The mainconsideration is the clay consistency, as the openings through thecutting face are calculated to restrict cut spoil ahead of the cutteruntil small enough to be able to fit through the vacuum chamber of thedrill head. When clay is soft it is easy to drill, but builds on itselfand can cause blockages if the correct cutter is not chosen.

A shale cutting face will be similar to the clay version, but faceopenings are modified to allow for front regrind of large chippedmaterial prior to vacuum extraction.

A rock cutting face generally comprises a cutter face with three, six ornine conical roller assemblies with peripheral openings (usually three)for cutting spoil extraction. Utilising multiple small diameter conicalrollers, each set of three are staggered in distance and angle from thefront face. The inner set of three cones being most forward, theintermediate set radially skewed from the inner at 60 degrees andsetback by 25-100% of the cut diameter, and the final set again radiallyskewed from the intermediate at 60 degrees to bring the inner conicalportion back in line with the radial centre-lines of the inner set ofcones, and setback from the intermediate face by another 25-100% of thecut diameter. Roller cutter face then has the benefit of continualsteering capability, increased stability in non-homogenous groundconditions, and increased chip rate resulting in less regrind time priorto vacuum extraction of spoil.

Downhole drilling technology has been using “tri-cone” rollers to cutrock for decades. They are available in a variety of grades—soft, mediumand hard formation. A tri-cone roller utilises three conical rollers,equispaced at 120 degrees, fitted with hard metal inserts each rotatingabout their own bearing shaft. The conical shape of each roller, taperedinto the centre of the cutting face, rotating about an axis skewed 60degrees forward in towards the centre of the cutter results in a fullflat face cut diameter. The resultant large flat cutting face is verydifficult to maintain stability in non-homogenous ground, and due to thesize of three rollers required to obtain the full cut diameter, theaxial distance traveled prior to any steering response is often half thecut diameter.

All front cutting rotors have pressure fluid ports. Holes are drilledradially to the centre of the cutter to coincide with the porting on thedrill shaft. Additional holes are drilled axially from both the frontand rear faces of the cutter. These holes are sized approx 2 mm diameterto allow extreme pressure at face for best cutting and mixing qualitieswith minimal pressure fluid usage. An internal chamfer on front ports toincrease surface area at opening only to allow for blockage ejection.Rear ports are directed back towards drill head to aid in clearing anyresidues from air channel and vacuum cavity.

All front cutting rotors have a central cavity for connection with thedrill shaft in the drill head. This cavity is either threaded with atrapezoidal or acme thread taking up onto a shoulder on the shaft, or ahollow hexagon for the quick connection arrangement used in conjunctionwith a front threaded cone and lock bolt. Both styles accommodate forthrough shaft and cutter pressure fluid transmission.

The drill head drives the front cutting rotor by way of the drill shaft.The front of the shaft is a male hexagonal drive, with 75-100% of acrossflats dimension of the hexagon in length, with a front threadedextension generally 50-75% of the across flats dimension of the hexagonin diameter, and 75-100% of the thread diameter in length.

The drill rod is radially drilled (eg 3×5 mm diameter holes at 120degrees) through the faces of the hexagonal final drive through to acentral larger axial port (eg 8 mm-12 mm diameter). This axial port isdrilled as a blind hole into the drill shaft, to the lengthcorresponding to the position of the front fluid bush. Here, anotherseries of smaller radial holes are drilled through to meet with theaxial port (eg 3×5 mm diameter holes at 120 degrees). These holes arepeened (eg 8-10 mm concave diameter) to eliminate any seal degradationfrom the rotating shaft.

The front fluid bearing bush encapsulates this mid-front section of thedrill rod and provides a centralised bearing location capable of highradial and thrust forces combined. The peened radial holes of the drillrod are longitudinally aligned with the internal radial pressure fluiddistribution groove of the fluid bearing bush.

This groove is in turn fed pressure fluid from radial drill holes (eg6×5 mm diameter holes equispaced at 60 degrees). Fluid cannot escape tothe rear of the fluid bush due to an energising U-cup seal placed at therear of M1 bearing module. Pressure fluid is proportionallydistributed—to the drill shaft axial port through to the front cuttingrotor, creating back pressure to distribute to the annulus area betweenthe outside diameter of the drill rod and the inside diameter of thefluid bush. This is achieved by high helix angle, low depth multi-startgrooves machined on the inside of the fluid bush from the front edge ofthe distribution groove to the front face of the fluid bush (egtriple-start, 20 mm pitch 0.5 mm deep grooves with 1.5 mm concaveradius). This pressure fluid is then channelled to a helical spiralgroove on the front face of the bush (eg single 10 mm pitch continuouslydecreasing right-hand 0.5 mm deep face groove with 1.5 mm concaveradius). This channelling effect essentially hydrostatically separatesthe shaft from the bush both radially and axially, to counteractsteering and thrust face forces. The relationship is linearlyproportional in that the higher the load, the harder the faces actagainst one another, providing a greater hydrostatic seal, which in turnacts to repel the two components. Hence we have a bearing, whichmechanically transfers load, provides a pressure fluid swivel, andcontinually lubricates and cools itself. This method allows a verystrong shaft construction with minimal stress riser points, andexcellent pressure fluid conveyance.

The drill head functions to drive the front cutting rotor by means of adrill rod. The bore hole position is monitored within the drill head bymeans of a laser set at the launch shaft indicating a position on atarget mounted in the drill head. A camera within the drill head isdirected at the target, and relays a video image to a video screenviewed by the machine operator. The operator controls any requiredsteering direction changes. Steering is achieved by altering theposition of the cutting face relative to the bore hole.

The prior art was to manufacture a cylindrical drill head, and movingthe cutting face. One steering method is to pivot the front portion ofthe drill head vertically and horizontally. Although effective insteering, this required the laser target to be situated a considerabledistance from the cutting face. The further rearward the laser targetposition, the further the distance is required to be drilled prior to anupdate of current bore face location.

Another steering method is to move the drill shaft within the drillhead. This has the advantage of being able to mount the laser targetfurther forward in the drill head, and therefore, providing a moreaccurate target to bore face position. However, the pivotal mounting ofthese steering mechanisms provides a weak steering with high failurerates and increased maintenance.

These past methods of steering are physically large and cumbersome, anddue to plumbing required to each hydraulic cylinder, makes this methodunsuitable to small diameter drill head design. The invention entailsconstruction of a modular drill head for increased strength and reducedsize.

The drill head is of a segmental modular design to minimise overall sizewhile achieving maximum strength and durability. Each module iscentralised and retained by the next module by male and female steppedspigots. Clamping of each module achieves angular alignment and axialclamping. Each module is designed for its particular purpose in thedrill head, and all hydraulic, fluid, air and vacuum channels areinterconnected by way of stepped face seals. It is this method ofconstruction that allows the use of integrated pressure porting,reliable bearing design, maximum vacuum area, good air channel ducting,maximum forward position of laser target area and plumb indicator forvisual head tilt indication.

The drill head and steering module for use in the microtunnelling systemhas a steering shell M2 mounted axially on the drive rod (22) in amanner to allow radial movement and having a plurality of radiallymounted pistons able to engage the inner surface of the steering shellM6 such that the control of the protrusion of the plurality of radiallymounted pistons controls the direction of the steering shell.

As shown particularly in FIG. 8, the plurality of radially mountedpistons is included in a circular steering module fitting around thedrill rod and having radial bores from which the radially mountedpistons protrude. The circular steering module includes a spoked wheeleffect with the radial bores extending at least partially along theradial extending spokes. Preferably cavities are between the spokes toallow axial pathways. The circular steering module includes ports nearthe radial centre and able to receive water or hydraulic fluid fordriving the pistons to protrude from the radial bores and engage theinner surface of the steering shell.

As shown in FIG. 2, the drill head includes a modular constructionhaving a plurality of circular disc like elements for axial alignmentand abutment and mounting within a cylindrical shell, wherein each ofthe circular disc like elements is created by direct bore constructionand the axial alignment and abutment creates continuous axial and radialchannels allowing fluid flow, vacuum waste return channel, and controlflows.

One of the circular disc like elements forms a bearing module M1 at thefront of the drill head with flow paths for providing axially extendingfluid jets to assist cutting and radially extending flow paths to assistaquaplaning bearings of the rotating cutting means.

One of the circular disc like elements forms a steering module M2 at thefront of the drill head with flow paths for providing axially extendingfluid jets to control protrusion of pistons to engage the outer cylinderand alter direction of the drill head.

One of the circular disc-like elements forms a spacer module M3 withinthe drill head with flow paths for providing axially extending flowpaths to adjacent modules.

One of the circular disc like elements forms a mounting module M4 at therear of the drill head with flow paths for providing axially extendingflow paths and able to form non rigid mounting of base of outercylinder.

The drill rod (22) and connected intermediate drill rods (23) are asteel rod drive shaft, with male and female hexagonal ends to effectconnection and resist torsional forces. The drill rod and connectedintermediate drill rods are retained within either end of the drill rodend plates by front and rear rod bush bearings. The drill rod andconnected intermediate drill rods are housed in an axially extendingtubular section (51) to separate the bearings from the spoil through thevacuum section. The axially extending tubular section drill stringhousing is located fully within the vacuum chamber, surrounded by thevacuum channel and vacuum cavities. It is this full surround by vacuumthat functions to absorb heat created by the rotating drill string,transferring it directly to the slurry and spoil cuttings and fluidreturning from the drill head, and in turn to the vacuum waste tank.

The laser beam used for drill head guidance travels through theprotected top air channel (52). It is the effective removal of heat andcreation of a stable laser environment that minimises otherwiseunavoidable hot-cold transitions at every drill rod connection. In pastdrill rods, these hot-cold transitions cause consecutive and culminatinglaser refraction, leading to an inaccurate borehole.

During connection the drill rods (23, 23) are pushed together. Thevacuum thrust adaptor has two conical combination pins (48) in the maledrill rod end plate (47) about the rod's longitudinal axis and centredvertically about the drive, and offset equidistant about the horizontalplane. These combination pins have a conical taper at the front andalign with two bores (49) in the female drill rod end plate (46) aboutthe rod's longitudinal axis. As the pins are further inserted, the drillrod is aligned to a horizontal plane; the drill rod and connectedhexagonal intermediate drill rods are aligned and further inserted untilthe two end plate faces are mating.

Consecutively during this alignment process, the toggles mounted to thefemale end plate are caused to pivot about the pivot bush axis, movingradially outwards from the end plate diameter, allowing the majordiameter of the combination pins past the toggles. Once the CombinationPins pass the major diameter, the toggles are allowed to spring back totheir original position, moving in between the combination pins and thefemale end plate, thus locking the connection, and allowing eitherthrust or pullback under load. Once the drill rod end plates are matedface to face, the vacuum and laser space are sealed due to theelastomeric seals inserted in the milled grooves of the female plate.

Referring to FIGS. 2, 4, and 5 the M1 bearing module comprises of acircular disc with a central stepped bore for the location of the frontfluid bearing bush. The housing is cross-drilled to divert an axialpressure fluid port originating to the side of the drill rod, connectedto a radially drilled port which in turn connects to a radial groove onthe inside of the central bore. Two additional smaller radialgrooves—one to the rear and one to the front of the channel grooveprovide housing for o-ring seals which completes this cavity and directsall pressure fluid through to the radial holes drilled through the fluidbush. The radial pressure cavity also connects to a vertical radial portfitted with a jetted plug, which directs some fluid to the Annulusbetween the steering ring and steering shell M6. At the rear of the M1bearing module is a self-energising u-cup seal retained by a soft metalbush to complete the front seal cavity.

As shown in FIGS. 2, 6, 7 and 8 the M2 steering module comprises acircular disc with a central bore through which the drill rod passes. Atthe top and to the sides are air channels. At the bottom is the vacuumcavity. There are four radial drillings, bores and counter boresequispaced around the circumference of the disc. Four independent oilports drilled axially from the rear of the housing and countersunk withface sealing enter the lower portion of the radial drilling in each ofthe four bores. These bores house the steering pistons with highpressure seals. With pressurised hydraulic oil entering any of thesecavities, the associated piston is forced radially outward providingforce to move the steering shell M6. The piston is retained fromejection from the housing by a stepped gland ring incorporating a pistonrod wiper and auxiliary seal which in turn is retained by an internalcirclip within the stepped bore.

The M6 steering shell comprises a hollow tubular section with a frontend stepped return section reducing in inside diameter then tapered bothinternally and externally towards the front. This front stepped returnis faced up against the front of M1 bearing module, and the main innerbore has full annular clearance around the circumference of the steeringring assembly allowing the shell to move about radially in anydirection. As one piston in the M2 steering module is actuated, the M6steering shell is forced radially and moves with the extending piston.As the opposing side of the M6 steering shell moves in towards thesteering ring assembly, the piston radially opposed to that actuated isin turn retracted, allowing for the next steering manoeuvre. The sameapplies to the other set of pistons acting about an axis at 90 degreesto the first set of pistons. This actuation on 2-cylinder movement axes,either independently or together allows the drill head to alter itsshaft and cutter position relative to the bored hole thus providingsteering control.

The hydraulically steered drill head has a fast system for changingcutting tooling. Rock capabilities have been enhanced with the design ofa rock roller system for the microtunnelling unit.

The drill head has been modified to accommodate the covered drill rodsystem and designed to allow for the introduction of automated steering.Drill head segmental design allows for strength and durability whilstenhancing the ability to maintain drill head positioning via hydraulicrams holding a position of one circular piece within a second circularring providing for maximum strength in minimal space.

The drill shaft must rotate freely under high loads, and pressure fluidmust be transferred to the drill face. The use of high-pressure fluidsout of the drill face allows for enhanced tooling life whilst alsogiving the ability to flush tacky ground.

The prior art was to retain the shaft within steel bearings, eithertapered roller, or ball bearings with needle thrust bearing. This solvedthe mechanical rotation issue, but brought with it a whole plethora ofassociated problems to do with sealing bearings from ingress of cuttingspoil and water, both ingredients deadly to bearings. Maintenance isincreased as seals and bearings have to be replaced regularly. If abearing was to seize, it would halt the complete drilling process, drillhead would have to be removed for overhaul, causing unplanned down-timeand site delays.

The prior art for pressure fluid transmission is with a pressure swivelassembly, which rotates about the shaft axis. The swivel constructionwould be tubular in design with two pressure seals axially opposed toretain a central pressure chamber within the swivel. A threaded inletport enters this central pressure chamber radially, flows around theaxis of the cavity, through a radial hole drilled in the drill shaft,then through an axial hole in the drill shaft to the front face. Thisdesign required external retention of the swivel housing to stop itrotating with the drill shaft, causing radial side-loads on one insideface, in turn, causing seal failure and therefore leakage. The seals hadto have a high preload to accommodate high pressure, and would weargrooves in the drill shaft, causing leakage. The swivel would be locatedbehind the target position, so any water spray from leaks would upsetvisual sight of target. Using pipe fittings from the swivel housing withelbows to bring hose in axially beside drill shaft meant size was toolarge to be used in small diameter drill heads, assembly and maintenanceof hose and fittings would be awkward at best.

The invention entails construction of a modular designed drill head,with integrated pressure fluid conveyance cavities. Further, theinvention includes the use of a fluid bearing bush to act as a frontdrill rod bearing and pressure swivel in one assembly. The fluid bearingbush is retained in the M1 bearing module by three grub screws(equispaced at 120 degrees). Pressure fluid directed to the distributiongroove in the M1 bearing module is sealed form escaping past the insideof the stepped bush bore and the outside diameter of the fluid bearingbush by means of two O-ring seals on each side of the distributiongroove. This M1 bearing module distribution groove is longitudinallyaligned with radial drill holes (eg 6×5 mm diameter holes equispaced at60 degrees) around the perimeter of the fluid bearing bush. These drillholes enter the inside diameter of the bush and are interconnected withan internal radial distribution groove within the fluid bearing bush.Fluid cannot escape to the rear of the fluid bush due to an energisingU-cup seal placed at the rear of M1 bearing module.

The fluid bearing bush encapsulates a mid-front section of the drill rodand provides a centralised bearing location capable of high radial andthrust forces combined. The peened radial holes of the drill rod arelongitudinally aligned with the internal radial pressure fluiddistribution groove of the fluid bearing bush.

Pressure fluid is proportionally distributed—through radial holes in thedrill shaft, connecting to an axial port through to the front cuttingrotor, creating back pressure to distribute to the annulus area betweenthe outside diameter of the drill rod and the inside diameter of thefluid bush. This is achieved by high helix angle, low depth multi-startgrooves machined on the inside of the fluid bush from the front edge ofthe distribution groove to the front face of the fluid bush (egtriple-start, 20 mm pitch 0.5 mm deep grooves with 1.5 mm concaveradius).

This pressure fluid is then channelled to a helical spiral groove on thefront face of the bush (eg single 10 mm pitch continuously decreasingright-hand 0.5 mm deep face groove with 1.5 mm concave radius). Thischannelling effect essentially hydrostatically separates the shaft fromthe bush both radially and axially, to counteract steering and thrustface forces. The relationship is linearly proportional in that thehigher the load, the harder the faces act against one another, providinga greater hydrostatic seal, which in turn acts to repel the twocomponents.

Hence we have a bearing, which mechanically transfers loads, provides apressure fluid swivel, and continually lubricates and cools itself. Thismethod allows a very strong shaft construction with minimal stress riserpoints, excellent radial and axial bearing loads, excellent impactresistance, excellent pressure fluid conveyance, minimal assembly andmaintenance costs, and is field replaceable.

The position of the target at the extreme front of the drill headultimately enhances the drills ability to be extremely accurate andresponsive to positional changes. The use of high-pressure fluids out ofthe drill face allows for enhanced tooling life whilst also giving theability to flush tacky ground. The ability to run drill fluids at thecutting face creates greater efficiencies within cutting and assists ourabilities through varied ground conditions. Front bearing combination ofhigh load axial and thrust bearing with a high-pressure fluid andintegrated lubrication system.

The drill rods are inserted and connected consecutively with the thrustmodule to allow bore hole progression while maintaining drill string,vacuum, air channel, hydraulic, pressure and data line connection. Thedrill rod transmits torque from the rotation unit mounted on the thrustmodule to the drill head at the bore face via a drill rod and connectedintermediate drill rods. The drill rod also transmits thrust from therotation unit mounted on the thrust module to the drill head at the boreface via a vacuum tube.

The prior art was to have the vacuum tube section aligned longitudinallywith the drill string, situated below it, generally to rest on theinvert of the borehole. This allows cutting spoil extraction by vacuum.

The vacuum tube has bearing bushes mounted at each end along the drillrod and connected intermediate drill rods axis to retain the drill rodand connected intermediate drill rods, and male and female cleats ateach end for connection by means of a manual pin inserted to two holeseither vertically or horizontally aligned. The drill string is exposed,causing possible operator injury from the rotating shaft. The connectionmethod with manual pin insertion is tedious, and pin extraction afterbore completion is difficult.

The manual connection method required clearance to allow manualconnection. This clearance between subsequent drill rods allows each rodto rotate slightly about its axis as a result of drill string rotationaltorque. This rotation, possibly only 1 degree per rod, extrapolates theerror the further the borehole. Final error over a 100 m bore could be a50-degree rotation, causing an inaccurate target position relative tothe start point. This target position is then potentially out by up to100 mm.

The borehole is not peripherally supported, causing ground collapse incertain ground conditions, thereby blocking laser and target view, andhalting drilling operation. The bearings are directly under the laserposition, causing hot sections at each end of the drill rod and a coolersection between the bearings. These hot-cold transitions causeconsecutive and culminating laser refraction, leading to an inaccurateborehole.

The microtunnelling system uses a casing mounted on the drill rod thatincludes at least two axially extending cavities or bores wherein liquidis axially transported along one of said axially extending cavities orbores under pressure to the drill head to assist drilling and resultingslurry is vacuum returned along the other of said axially extendingcavities or bores. However as drill rods are fully enclosed, andslightly smaller than the drill head diameter allowing themicrotunnelling machine to be effective in collapsing ground conditions,under water table, soft or hard ground. The vacuum or slurry spoilextraction volume within the drill rod provides minimum restriction toincrease productivity and length of lines achievable. With all movingcomponents enclosed, the drill rod is safer to use.

Rotation within vacuum or slurry spoil eliminates heat from bearings,minimising laser distortion and wear and tear to the equipment. Enclosedlaser space for stability of beam. Provides airflow to equalisetemperature and humidity, more accurate operation. Automatic alignmentsystem speeds and simplifies operation. Automatic clamping system, forpositive joining, withstands full load in both forward and reversedirections. Clamping system maintains strong sealing of vacuum. Fullyencapsulated hose and dataline pocket, protecting sensitive data andpressure lines.

The pullback extraction reamer is used to increase the size of amicrotunnelled bore hole. This is advantageous for operators as one sizemicrotunnelling drill head and drill rods can be used in conjunctionwith a pullback extraction reamer in various bore sizes, whilemaintaining good productivity. Once the drill head reaches the receptionshaft, the drill head is removed from the end of the drill rod andreplaced by the pullback extraction reamer. The product pipe to beinstalled can be coupled to the pipe pullback adaptor mounted on therear. Drilling is now commenced in reverse, or pullback mode. The drillstring is coupled to a drive spur gear that rotates three planetarygears fixedly mounted to the vacuum thrust plate. The spur gears aremeshed inside an internal ring gear that is fixed to the cutter hub,allowing the cutter hub to rotate at a lower speed but higher torquethan its input drive. The cutter hub is mounted to the pipe pullbackadaptor by way of thrust and radial bearings. This embodiment allows thedrill rod and pullback pipe to remain rotatably fixed and the reamercutter hub can rotate about the longitudinal axis at a greater torque.The cutter hub is typically concave within its cutting face, so that asit is pulled back through the ground, slurry and spoil are offered tothe vacuum or slurry channel entrance for evacuation.

It should be understood that the above description is of a preferredembodiment and included as illustration only. It is not limiting of theinvention. Clearly a person skilled in the art without any inventivenesswould understand variations of the microtunnelling system and apparatusand such variations are included within the scope of this invention asdefined in the following claims.

1-37. (canceled)
 38. A backreaming apparatus comprising: a plurality ofintermediate drill rods that can be connected together to form a stringof intermediate drill rods, each intermediate drill rod including adrive shaft rotatably mounted within a casing, the casings of theintermediate drill rods being connected together by connections thatallow a pull-back load to be transferred through the casings of theintermediate drill rods, and the drive shafts of the intermediate drillrods being connected by connections that allow torque to be transferredthrough the string of intermediate drill rods; a backreamer positionedadjacent a first end of the string of intermediate drill rods; and anexternal drive positioned at a second end of the string of drill rodsfor applying a pull-back load to the string of intermediate drill rodsand for applying torque to the string of intermediate drill rods, thetorque being transferred to the backreamer by the drive shafts of theintermediate drill rods and the pull-back load being transferred to thebackreamer by the casings of the intermediate drill rods.
 39. Thebackreaming apparatus of claim 38, wherein the intermediate drill rodsdefine a slurry channel for removing slurry from a hole beingbackreamed, the slurry channel having an inlet adjacent the backreamerand an outlet connected to a source of vacuum.
 40. The backreamingapparatus of claim 38, wherein torque is transferred from the driveshafts to a backreamer cutter hub by a drive gear that rotates aplurality of planetary gears.
 41. The backreaming apparatus of claim 39,wherein the backreamer includes a cutter hub having a cutting face thatis concave and configured to offer spoils to the inlet of the slurrychannel.
 42. The backreaming apparatus of claim 40, wherein theplanetary gears have rotation points that are mounted to a thrust platethat is fixed relative to the casings of the intermediate drill rods.43. The backreaming apparatus of claim 38, wherein the casings of theintermediate drill rods do not rotate as the backreamer is rotated bythe drive shafts.
 44. The backreaming apparatus of claim 42, wherein acylindrical pipe attachment housing is connected to the thrust plate.45. The backreaming apparatus of claim 38, wherein the casings includeouter cylindrical portions adapted to support a bore being backreamed,wherein the casings define first and second separate axial channels thatextend though the string of intermediate drill rods and are fullyenclosed within the casings.
 46. The backreaming apparatus of claim 45,wherein the casings include end plates positioned at first and secondends of the casings, and wherein the drive shafts are rotatably retainedwithin the end plates by bearings.
 47. The backreaming apparatus ofclaim 46, wherein the end plates define alignment holes offset fromcenter axes of the intermediate drill rods, wherein the intermediatedrill rods also include projections that project axially outwardly fromthe end plates and that axially slide into the alignment holes ofadjacent intermediate drill rods to provide rotational alignment betweenthe adjacent intermediate drill rods.
 48. The backreaming apparatus ofclaim 47, wherein the intermediate drill rods include latches positionedadjacent the alignment holes for engaging the axial projections toretain the axial projections within alignment holes, and whereininteraction between the axial projections and the latches allows thepull-back load to be transferred through the casings of the intermediatedrill rods.
 49. The backreaming apparatus of claim 38, wherein thecasings define axially extending cavities having outwardly facing opensides that extend along the lengths of the casings.
 50. The backreamingapparatus of claim 38, wherein the casings of the intermediate drillrods are aligned by male and female structures that are offset fromcenter axes of the intermediate drill rods and that mate together whenthe intermediate drill rods are moved axially together in an end-to endrelationship, wherein the intermediate drill rods also include latchesthat engage the male structures when axially inserted into the femalestructures, and wherein interaction between the latches and the malesstructures allows pull-back load to be transferred axially through thecasings of the intermediate drill rods.
 51. The backreaming apparatus ofclaim 50, wherein each of the casings defines first and second axiallyextending cavities that extend from a first end to a second end of eachof the intermediate drill rods, and wherein the male and femalestructures ensure alignment of first cavities and also ensure alignmentof the second cavities when the intermediate drill rods areinterconnected.
 52. The backreaming apparatus of claim 50, wherein themale structures include pins and the female structures include sockets.53. The backreaming apparatus of claim 38, wherein the backreamer issized for use in installing pipe having a diameter less than 600millimeters.
 54. The backreaming apparatus of claim 38, wherein theintermediate drill rods define a central axis, wherein the drive shaftsare aligned along the central axis, wherein the casings are also alignedalong the central axis, wherein the casings define cavities that extendaxially though the casings at a location offset from the central axis,wherein the intermediate drill rods include axial projections thatprojects axially outwardly from first ends of the casing at locationsoffset from the central axis, the intermediate drill rods also includingprojection receivers in second ends of the casing at locations offsetfrom the central axis, the axial projections being axially receivedwithin the projection receivers to provide alignment between thecavities of adjacent intermediate drill rods.
 55. The backreamingapparatus of claim 54, wherein at least two axial projections areprovided at each of the first ends of the intermediate drill rods and atleast two projection receivers are provided at each of the second endsof the intermediate drill rods.
 56. The backreaming apparatus of claim55, wherein the axial projections at the first ends of the intermediatedrill rods are separated by about 180 degrees, and the projectionreceivers at the second ends of the intermediate drill rods areseparated by about 180 degrees.
 57. The backreaming apparatus of claim54, further comprising latches provided at the second ends of theintermediate drill rods for retaining the axial projections within theprojection receivers and for allowing pull-back load to be transferredaxially between adjacent intermediate drill rods.
 58. A method forinstalling product comprising: drilling a hole by pushing a rotatablecutting face through the ground by progressively adding drill rods to astring of drill rods, wherein the drill rods include outer casings fortransferring thrust to the cutting face and inner drive shafts fortransferring torque to the cutting face; monitoring the direction of thedrill string using a laser directed through the string of drill rods toa target located adjacent the cutting face; after the hole has beendrilled, replacing the cutting face with a backreamer; and enlarging thehole by pulling the backreamer back through the hole with the casingswhile applying torque to the backreamer with the drive shafts.
 59. Themethod for installing product of claim 58, further comprising attachingproduct behind the backreamer and pulling the product into the hole asthe hole is backreamed.
 60. The method of claim 59, wherein the productis pipe having a diameter less than 600 millimeters.